This invention relates to video processing and particularly to spatial video processing. An important example is the creation of additional lines through spatial interpolation.
A spatial interpolation filter has a filter aperture comprising an appropriate number and arrangement of neighbouring pixels and takes a weighted sum of contributions from pixels within that filter aperture. It has previously been recognised that such a filter can introduce smearing if there are pronounced slopes in the picture material. It is instructive to take the example of a striped image. A filter designed to take weighted sums of contributions along a vertical pixel line will be expected to function correctly as long as the stripes in the image are almost exactly horizontal. Essentially, weighted averages are taken along the stripe and therefore from pixels which are similar. If diagonal stripes are encountered, a weighted sum of contributions along a vertical pixel line will now mix pixels which are from different stripes and which may be very different. Smearing of the image is likely to result.
One approach to this problem, particularly where dealing with interlaced material, is to avoid using spatial interpolation and instead to utilise temporal interpolation. This might work well for stationary pictures but is unlikely to be a satisfactory solution where there is movement between successive pictures.
It has been proposed to improve spatial interpolation by measuring pronounced slopes in the picture material and then rotating the filter aperture into alignment with the measured slope. In the example of diagonal stripes, the angle of the stripe is measured and the filter aperture rotated so that weighted contributions are taken along the stripe, from pixels that remain similar. This proposal can be very helpful if the slope is measured accurately and the filter rotated into precise alignment. This can however be a complex operation. Moreover, if the filter is rotated by the wrong amount, the results can be as bad as, or sometimes worse than, if the aperture remained static. It is also a factor that if the filter aperture is to be able to accommodate shallow slopes (and it is shallow slopes that produce the most objectionable artefacts), a very wide filter aperture is required.
It is an object of this invention to provide improved video processing in which slopes are accommodated with greater tolerance to errors or variations in slope angles.
It is an object of one form of this invention to meet the performance of prior art slope processing with substantially smaller filter apertures.